What is a singular noun?
A singular noun refers to one thing
All nouns are either plural or singular. Plural nouns refer to multiple things and often end in -s or -es (ex. dogs, cats, ponies, crates, pies, etc.). Singular nouns refer to one thing (ex. circle, heart, pastry, book, etc.).
Some nouns are irregular, meaning that their plural forms do not end in -s or -es as most plural nouns do. Some examples are radii (plural of radius), mice (plural of mouse), and children (plural of child). They are still plural because they refer to more than one thing.